The complement system is part of the host innate immune system involved in lysing foreign cells, enhancing phagocytosis of antigens, clumping antigen-bearing agents, and attracting macrophages and neutrophils. The complement system is divided into three initiation pathways—the classical, lectin, and alternative pathways—that converge at component C3 to generate an enzyme complex known as C3 convertase, which cleaves C3 into C3a and C3b. C3b associates with C3 convertase mediated by CFB and results in generation of C5 convertase, which cleaves C5 into C5a and C5b, which initiates the membrane attack pathway resulting in the formation of the membrane attack complex (MAC) comprising components C5b, C6, C7, C8, and C9. The membrane-attack complex (MAC) forms transmembrane channels and disrupts the phospholipid bilayer of target cells, leading to cell lysis.
In the homeostatic state, the alternative pathway is continuously activated at a low “tickover” level as a result of activation of the alternative pathway by spontaneous hydrolysis of C3 and the production of C3b, which generates C5 convertase.